Hydrogen sulfide is environmently undesirable and thus its decomposition is desired or required in many instances. Hydrogen sulfide pollution and other problems caused thereby are of significant concern to many energy related industries such as crude oil refining, coal gasification, and the supplying of natural gas and geothermal energy.
There are various prior art methods for hydrogen sulfide (H.sub.2 S) disposal. The oil refining industry, for example, uses a method known as the Claus process. In this process, H.sub.2 S is burned yielding sulfur dioxide and water. The sulfur dioxide in turn is further reacted with H.sub.2 S to yield sulfur, water and heat. The reactions are highly exothermic and this heat can be utilized to generate high grade process steam for hydrocracking, another step in the oil refining process. The equipment and processing plant necessary for practicing the Claus method of H.sub.2 S decomposition however, represents a very large capital investment. Therefore, except for a process such as oil refining where a use for the heat generated by the Claus process is present, the capital investment can not be economically justified. Other disadvantages of the Claus process are that its hydrogen sulfide conversion efficiency is only about 92 percent and other pollutants such as CS.sub.2 and COS are normally also produced by the Claus process. Thus, for many applications, H.sub.2 S decomposition by the Claus method is not a practical solution.
Hydrogen sulfide contamination perhaps presents the most troublesome problems for suppliers and users of natural gas. Natural gas burns more efficiently and with less pollutants after removal of hydrogen sulfide. Also, hydrogen sulfide is quite corrosive and its inclusion in natural gas corrodes pipelines used for transporting the gas and can poison the catalyst of the burners of the devices in which the natural gas is ultimately utilized. Poisoning occurs when the active sites of the catalyst become inactivated by poisonous species resulting from the H.sub.2 S impurities contained in the natural gas. Once such active sites become inactivated, they are no longer available to act as a catalyst for the desired reaction resulting in decreased conversion efficiencies.
One prior art hydrogen sulfide removal process which has been proposed for decontaminating a hydrocarbon gas, such as natural gas, is disclosed in U.S. Pat. No. 3,409,520. The patent discloses an electrolysis cell having an electrolyte therein and an anode and a cathode in contact with the electrolyte with the electrodes being connected to an external source of electric power. The hydrogen sulfide-hydrocarbon gas mixture is introduced into the electrolysis cell and into contact with the anode which preferably is constructed of a porous carbon material impregnated with a catalyst such as platinum. In operation, an externally generated electric current is passed through the electrolyte between the anode and the cathode in order to electrolytically oxidize sulfide ions at the anode to form a sulfur oxidation product of the sulfide ions which may be free sulfur, certain polysulfides, or both. At the same time, hydrogen ions are electrolytically reduced to form free hydrogen at the cathode. The sulfur product and hydrogen gas are separately withdrawn from the electrolysis cell and disposed of as desired. The hydrocarbon gas is also withdrawn separately from the cell yielding an at least partially decontaminated gas.
Other methods of decomposing hydrogen sulfide are disclosed in U.S. Pat. Nos. 4,314,983, 3,994,790, 3,266,941, 3,249,522, and 4,320,180. The last two noted patents operate not only to decompose H.sub.2 S, but in so doing produce electricity. U.S. Pat. No. 3,249,522 operates as a fuel cell employing anodic oxidation of hydrogen sulfide and cathodic reduction of oxygen or air. The fuel cell of that invention operates in a similar manner to a conventional fuel cell wherein hydrogen is utilized as the oxidant and reacts with the electrolyte's hydroxyl ions in the presence of the anode catalyst to form water and release electrons. With H.sub.2 S utilized as a fuel, sulfide ions are oxidized at the anode releasing electrons: S.sup.2- .fwdarw.S.sup.O +2e.sup.-. The sulfur produced reacts with other sulfide ions to form disulfide and polysulfide ions: S.sup.O +S.sup.2- .fwdarw.S.sub.2.sup.2-. The anode materials disclosed by the patent include carbon which can have impregnated or disposed thereon conventional fuel cell catalysts such as one or more metals of Groups Ib, Vb, VIb and VIII of the periodic table. One problem with this fuel cell is that when free sulfur is formed as an anodic product, the finely divided precipitate of sulfur can enter the pores of the porous gas diffusion anode and inactivate it causing an increase in the anode polarization and an increase in the resistnce across the anode-electrolytic interface thereby reducing the electrical output.
One attempt to overcome this problem in a fuel cell utilizing H.sub.2 S is proposed in U.S. Pat. No. 4,320,180. Therein is disclosed a fuel cell, in which a redox couple is used as the negative electrolyte which is oxidized at the fuel cell anode and then subjected to reduction outside the electrolytic cell by reaction with H.sub.2 S in a remotely located reaction column. The sulfur formed in the reaction column is removed from the electrolyte before the reduced electrolyte is recirculated back into the fuel cell. Consequently, the sulfur is prevented from inactivating the platinum or Raney nickel anode catalyst of the fuel cell.
One drawback of the electrolytic H.sub.2 S decomposition and fuel cell devices of the prior art, concerns the particular materials utilized for the anode catalyst. Often proposed for such use are noble metal containing materials, such as platinum, because of their high catalytic efficiency. Such metals, however, are not only quite expensive, but also are very susceptible to poisoning, reducing their commercial acceptability. The poisoned catalyst will not purify the contaminated hydrogen sulfide gas to the extent required, consume a greater amount of energy to remove a like amount of contaminants, and in the case of noble metal catalysts is quite expensive to replace.
Other drawbacks of the prior art anode catalysts result from the fact that they are generally based upon a crystalline structure. In a crystalline structure the catalytically active sites which provide the catalytic effect of such materials result primarily from accidently occurring, surface irregularities which interrupt the periodicity of the crystalline lattice. A few examples of such surface irregularities are dislocation sites, crystal steps, surface impurities and foreign adsorbates. A major problem with a crystalline structure is that the number of such irregularities forming the catalytically active sites are relatively few and occur only on the surface of the crystalline lattice. This results in the catalytic material having a density of catalytically active sites which is relatively low. Thus, the catalytic efficiency of the material and the device in which it is utilized is substantially less than that which would be possible if a greater number of catalytically active sites were available for the hydrogen sulfide decomposition or other desired reaction.
Thus, high catalytic efficiency from a relatively low cost material which is resistant to poisoning and stable in the H.sub.2 S cell environment, remain as desired results which must be attained before there will be widescale commercial utilization of devices of the type to which this invention relates.